This study is to evaluate the effectiveness of regenerative endodontics therapy in single-rooted permanent teeth with pulp necrosis，and compare the clinical efficacy of platelet rich fibrin (PRF) and blood clot (BC) as scaffolds.

Regenerative endodontics therapy (RET), based on the principles of tissue engineering, manipulates stem cells, scaffold, and bioactive growth factors to achieve the functional reconstruction of pulp tissue. RET has been recommended strongly as an alternative in treating immature permanent teeth with necrotic pulp in recent years, while root canal therapy (RCT) has been extensively applied in the treatment of mature permanent teeth with necrotic pulp. RCT contains the chemomechanical preparation of the infected root canals to eliminate the microorganisms, and root canal obturation with filling materials to reduce and prevent microbial contamination in root canal system. However, the root canal filling materials, such as Gutta-percha, are inert materials which fail to recover the physiological function of the pulp. As RET has been proved curative for immature permanent teeth diagnosed with pulp necrosis, doctors and researchers try to extend its application in the mature ones. Encouragingly, several RET cases for mature teeth reported in the literature have shown positive clinical outcomes. However, it still lacks long-term and well-designed randomized clinical trial with large sample size and following standardized protocol, hence this study is conducted to make up for it to achieve high-level evidence.

In the process of RET, the scaffold can provide nutrition and space, which are essential for the proliferation and differentiation of stem cells. Therefore, it is a crucial step to select a high-quality material as the scaffold. In many studies of RET procedures, blood clot (BC), generated by provoking apical bleeding into the root canal, is regarded as a scaffold. Nevertheless, it remains a common problem that operators may fail to induce apical bleeding or achieve adequate blood volume. Recent studies suggest that patelet rich fibrin (PRF), the second-generation platelet concentrate, can provide a fibrin network full of cytokines and growth factors, which may improve the desired biological outcome. Accordingly, we assumed that PRF could improve the curative effect for mature teeth in RET. The purpose of this study is to compare the clinical efficacy of PRF and BC as scaffolds in RET for the mature permanent tooth with pulp necrosis.

Inclusion Criteria:

• Intended to participate with this study, and provide informed assent/consent.
• Aged 10 and 59 years.
• At least one mature permanent teeth diagnosed as pulp necrosis with or without periapical lesion, and CBCT showed the teeth has single root and single root canal.

Exclusion Criteria:

• Allergic to any medications or materials necessary to complete the procedures.
• The teeth with severe coronal defect, of which pulp space is needed for post/core final restoration.
• Non-restorable teeth.
• The teeth with root fracture or vertical root fractures.
• Concurrent signs of other pathological root resorption.
• Patients with periodontitis.
• Patients with dental dysplasia or other oral genetic disorders.
• Women who are pregnant or lactating, or women who plan to become pregnant in the subsequent 2 years.
• Patients with dental phobia.
• Patients with mental disorders.
• Patients with a history of systemic diseases that may alter immune function.
• Patients with medical conditions and/or receiving medications that would affect patients' body's ability of healing or blood clotting.
• Participants who are participating in or had participated in other clinical studies within the prior 3 months.